Energy transfer to the reaction centres in bacterial photosynthesis

From a combined study of (1) bacteriochlorophyll fluorescence lifetimes, (2) relative yields and (3) differential absorption changes corresponding to the reaction centres photooxidation, the absolute values of fluorescence lifetimes and quantum yields for two bacteriochlorophyll fractions have been calculated. The main bacteriochlorophyll fraction (80–90%) serving as a light-gathering antenna for reaction centresP ₈₉₀ is characterized by dark values of fluorescence lifetimes of the order of 10^–11 sec and fluorescence yields of 10^–3. The remaining part of the bulk pigment, not associated withP ₈₉₀ as far as excitation energy transfer is concerned, has an approximately constant fluorescence yield of about 5–8% and lifetime of about 10^–9 sec. Basing on these results, excitation jump times and intermolecular coupling energies were estimated to be 10^–13 sec and 10^–2 ev respectively. The conclusion is made that excitation energy transfer in the main part of bacteriochlorophyll occurs by the exciton mechanism at moderate intermolecular energies.     

Probing Photosynthesis on a Picosecond Time Scale: Evidence for Photosystem I and Photosystem II Fluorescence in Chloroplasts

Fluorescent emission kinetics of isolated spinach chloroplasts have been observed at room temperature with an instrument resolution time of 10 ps using a frequency doubled, mode-locked Nd:glass laser and an optical Kerr gate. At 685 nm two maxima are apparent in the time dependency of the fluorescence; the first occurs at 15 ps and the second at 90 ps after the flash. The intervening minimum occurs at about 50 ps. On the basis of theoretical models, lifetimes of the components associated with the two peaks and spectra (in escarole chloroplasts), the fluorescence associated with the first peak is interpreted as originating from Photosystem I (PSI) (risetime ≤10 ps, lifetime ≤10 ps) and the second peak from Photosystem II (PSII) (lifetime, 210 ps in spinach chloroplasts and 320 ps in escarole chloroplasts). The fact that there are two fluorescing components with a quantum yield ratio ≤0.048 explains the previous discrepancy between the quantum yield of fluorescence measured in chloroplasts directly and that calculated from the lifetime of PSII. The 90 ps delay in the peak of PSII fluorescence is probably explained by energy transfer between accessory pigments such as carotenoids and Chl a. Energy spillover between PSI and PSII is not apparent during the time of observation. The results of this work support the view that the transfer of excitation energy to the trap complex in both photosystems occurs by means of a molecular excitation mechanism of intermediate coupling strength. Although triplet states are not of major importance in energy transfer to PSII traps, the possibility that they are involved in PSI photochemistry has not been eliminated.     

Structure-based calculation of multi-donor multi-acceptor fluorescence resonance energy transfer in the 4×6-mer tarantula hemocyanin

Hemocyanins are oxygen carriers of arthropods and molluscs. The oxygen is bound between two copper ions, forming a Cu(II)-O₂ ^2–-Cu(II) complex. The oxygenated active sites create two spectroscopic signals indicating the oxygen load of the hemocyanins: first, an absorption band at 340 nm which is due to a ligand-to-metal charge transfer complex, and second, a strong quenching of the intrinsic tryptophan fluorescence, the cause of which has not been definitively identified. We showed for the 4×6-mer hemocyanin of the tarantula Eurypelma californicum that the fluorescence quenching of oxygenated hemocyanin is caused exclusively by fluorescence resonance energy transfer (FRET). The tarantula hemocyanin consists of 24 subunits containing 148 tryptophans acting as donors and 24 active sites as acceptors. The donor–acceptor distances are determined on the basis of a closely related crystal structure of the horseshoe crab Limulus polyphemus hemocyanin subunit II (68–79% homology). Calculation of the expected fluorescence quenching and the measured transfer efficiency coincided extraordinary well, so that the fluorescence quenching of oxygenated tarantula hemocyanin can be completely explained by Förster transfer. This results explain for the first time, on a molecular basis, why fluorescence quantum yield can be used as an intrinsic signal for oxygen load of at least one arthropod hemocyanin, in particular that from the tarantula.     

Non-adiabatic electronic energy transfer from the (3CTRu(bpy)2(CN)2 excited state to Cr(III) complexes

The quenching of the luminescence lifetime of cis-Ru(bpy)₂(CN)₂ (bpy = 2,2′-bipyridine) by complexes of the cis- and trans-Cr(en)₂(XY)⁺ families (en = ethylenediamine; X and Y = F, Cl, Br, NCS, ONO) has been studied in aqueous solution and the results obtained have been discussed together with those previously reported for the quenching of the Ru(bpy)₃²⁺ luminescene by the same Cr(III) complexes. Experimental results and theoretical considerations show that the quenching process occurs by exchange electronic energy transfer. Since in all cases the process is sufficiently exoergonic to make up for the small intrinsic barriers, the lowest diffusion values of the quenching constants indicate a non-adiabatic behavior. The degree of adiabaticity of the energy transfer process is larger for the neutral Ru(bpy)₂(CN)₂ donor than for the positively charged Ru(bpy)₃²⁺ donor. The X and Y ligands can be ordered in the following adiabaticity series: ONO^?, F^? < Cl^? <NCS^? <Br^?. The geometry of the acceptor is a discriminating parameter only for energy transfer from the charged donor. These results show that the electronic term of exchange energy transfer in non-adiabatic processess is governed by a delicate balance of factors related to the composition and structure of the encounter complex [1].     

Measurement of the extent of electron transfer to the bacteriopheophytin in the M-subunit in reaction centers of Rhodopseudomonas viridis

We have measured the extent of flash-induced electron transfer from the bacteriochlorophyll dimer, P, to the bacteriopheophytin in the M-subunit, H_M, in reaction centers of Rhodopseudomonas viridis. This has been done by measuring the transient states produced by excitation of reaction centers trapped in the PH_L ^–H_M state at 90 K. Under these conditions the normal forward electron transfer to the bacteriopheophytin in the L-subunit, H_L, is blocked and the yield of transient P⁺H_M ^– can be estimated with respect to the lifetime of P^*. Under these conditions flash induced absorbance decreases of the bacteriochlorophyll dimer 990 nm band suggest that a transient P⁺ state is formed with a quantum yield of 0.09±0.06 compared to that formed during normal photochemistry. These transient measurements provide an upper limited on the yield of a transient P⁺ H_M ^– state. An estimate of 0.09 as the yield of the P⁺ H_M ^– state is consistent with all current observations. This estimate and the lifetime of P^* suggest that the electron transfer rate from P^* to H_M, k_M, is about 5 × 10⁹ sec^–1 (_M = 200ps). These measurements suggest that the a branching ratio k_L/k_M is on the order of 200. The large value of the branching ratio is remarkable in view of the structural symmetry of the reaction center. This measurement should be useful for electron transfer calculations based upon the reaction center structure.     

Excited-state equilibration in a meso-/microporous material-hosted bichromophoric [Ruthenium (2,2′-bipyridine)3]: Reversible energy transfer and photosensitized electron pumping

A bichromophoric pyrene-appended [Ru(bpy)₃]²⁺-type complex (bpy = 2,2′-bipyridine) showing rapid, reversible intramolecular energy transfer processes leading to an excited-state equilibration in homogeneous solutions was introduced into different photoinert supports, namely MCM-41, ethylene-bridged periodic mesoporous organosilica (PMO), and zeolite NaY. Its photophysical behaviour in each of these supports is compared with its behaviour in solution and with that of an appropriate model lacking the pyrene chromophore, [Ru(bpy)₂dmb]²⁺. Results suggest that the excited-state equilibration process which is operational in homogeneous solutions leading to long-luminescence lifetimes, is equally observed in all the different supports. A diminished oxygen sensitivity and prolonged luminescence lifetime was recorded for all the complexes included with respect to the analogous species in solution. The adsorbed pyrene-appended [Ru(bpy)₃]²⁺ complex is also shown to participate in photosensitized electron pumping from zeolite NaY to a size-excluded methylviologen in solution more efficiently than the adsorbed parent complex.     

A Comparative Study of Interaction of Tetracycline with Several Proteins Using Time Resolved Anisotropy,Phosphorescence, Docking and FRET

A comparative study of the interaction of an antibiotic Tetracycline hydrochloride (TC) with two albumins, Human serum albumin (HSA) and Bovine serum albumin (BSA) along with Escherichia Coli Alkaline Phosphatase (AP) has been presented exploiting the enhanced emission and anisotropy of the bound drug. The association constant at 298 K is found to be two orders of magnitude lower in BSA/HSA compared to that in AP with number of binding site being one in each case. Fluorescence resonance energy transfer (FRET) and molecular docking studies have been employed for the systems containing HSA and BSA to find out the particular tryptophan (Trp) residue and the other residues in the proteins involved in the binding process. Rotational correlation time (θ_c) of the bound TC obtained from time resolved anisotropy of TC in all the protein-TC complexes has been compared to understand the binding mechanism. Low temperature (77 K) phosphorescence (LTP) spectra of Trp residues in the free proteins (HSA/BSA) and in the complexes of HSA/BSA have been used to specify the role of Trp residues in FRET and in the binding process. The results have been compared with those obtained for the complex of AP with TC. The photophysical behaviour (viz., emission maximum, quantum yield, lifetime and θ_c) of TC in various protic and aprotic polar solvents has been determined to address the nature of the microenvironment of TC in the protein-drug complexes.     

Effects of bis(salicylidene)trimethylenediamine and 2,2′-bipyridyl on luminescence and extraction of tris(pivaloyltrifluoroacetonato)Eu(III)

The UV, excitation and luminescence spectra of EuA₃B to be the extracted species as well as the extraction of Eu(III) with pivaloyltrifluoroacetone, HA, and/or Lewis bases, B (2,2′-bipyridyl, bpy, and bis(salicylidene)trimethylenediamine, H₂saltn) into CHCl₃ were measured. The results are summarized: the stability constants of EuA₃bpy and EuA₃H₂saltn complexes are 5.85 ± 0.05 and 2.95 ± 0.06 as , respectively. The present results suggest that because of intramolecular hydrogen bonding, the stability and luminescence of the H₂saltn complex including the quantum yield are smaller than those of the bpy complex. The weaker luminescence is also concerned with the fact that the less stable complexes easily dissociate in solvents to diminish the essential concentration.     

The effect of pressure on the photochemistry of tris(bipyridyl)chromium(III) ion

The quantum yield for the photoaquation of Cr(bpy)₃³⁺ in basic medium decreases with increasing pressure, with an apparent volume of activation of 3.8 ± 1.0 ml mol⁻¹. From this value and that associated with the phosphorescence lifetime, the volumes of activation for non-radiative decay to the ground state and for formation of photoproduct are derived as −1.6 and +2.9 ml mol⁻¹, respectively. The latter value is consistent with either an associative process with water entering from pockets between the ligands or a dissociative process involving one or both bonds to a bipyridyl ligand.     

Synthesis and luminescence properties of novel 8‐hydroxyquinoline derivatives and their Eu(III) complexes

Six novel 8‐hydroxyquinoline derivatives were synthesized using 2‐methyl‐8‐hydroxyquinoline and para‐substituted phenol as the main starting materials, and were characterized by ¹H nuclear magnetic resonance (NMR), mass spectrometry (MS), ultraviolet (UV) light analysis and infra‐red (IR) light analysis. Their complexes with Eu(III) were also prepared and characterized by elemental analysis, molar conductivity, UV light analysis, IR light analysis, and thermogravimetric–differential thermal analysis (TG–DTA). The results showed that the ligand coordinated well with Eu(III) ions and had excellent thermal stability. The structure of the target complex was EuY^1–6(NO₃)₃.2H₂O. The luminescence properties of the target complexes were investigated, the results indicated that all target complexes had favorable luminescence properties and that the introduction of an electron‐donating group could enhance the luminescence intensity of the corresponding complexes, but the addition of an electron‐withdrawing group had the opposite effect. Among all the target complexes, the methoxy‐substituted complex (–OCH₃) had the highest fluorescence intensity and the nitro‐substituted complex (–NO₂) had the weakest fluorescence intensity. The results showed that 8‐hydroxyquinoline derivatives had good energy transfer efficiency for the Eu(III) ion. All the target complexes had a relatively high fluorescence quantum yield. The fluorescence quantum yield of the complex EuY³(NO₃)₃.2H₂O was highest among all target complexes and was up to 0.628. Because of excellent luminescence properties and thermal stabilities of the Eu(III) complexes, they could be used as promising candidate luminescent materials.     

Flow of 14C-methanol via assimilatory and dissimilatory sequences with yeast in presence of glucose

Experiments were performed to reveal the extent to which individual heterotrophic substrates of a mixture contribute to the overall carbon and energy metabolism. For this reason Hansenula polymorpha MH 20 was chemostatically (C-limited) cultivated at different growth rates on mixtures of methanol and glucose fed at proportions of 3:1 and 1:3 (in weight units), respectively. The distributions of ¹⁴C-carbon from methanol in biomass as well as carbon dioxide (and supernatant) fractions were determined. From these results it followed, firstly, that energy derived from methanol dissimilation was used in part for the incorporation of glucose carbon, resulting in carbon conversion efficiencies for this substrate equivalent to yield coefficients of 0.61–0.69 g/g. Secondly, the growth yield data revealed that the efficiency of methanol conversion had to be increased in order to account for the experimentally determined yield figures. This was further confirmed by theoretical treatment of the growth yield data which showed that these could only be obtained if P/O-quotients for methanol conversion similar to those for glucose, i.e. 2.0–2.5, were considered. The latter property was regarded as the main reason for the observed improvement of growth yield accompanying the simultaneous utilization of methanol and glucose in this yeast.Abbreviations ATP_M,a ATP required for incorporation of assimilated methanol at a given P/O-quotient - ATP_M,d ATP generated from dissimilated methanol at a given P/O-quotient - G and M glucose and methanol; respectively (the indices u, a, d and e mean utilized, assimilated, dissimilated and incorporated by excess energy, respectively) - PGA 3-phosphoglyceric acid - Y _G ^app apparent growth yield on glucose in presence of methanol - Y _G ^P/O theoretical growth yield on glucose at a given P/O-quotient     

Binding of Cu (II), Tb (III) and Fe (III) to chicken ovotransferrin

The kinetics of binding of Cu (II), Tb (III) and Fe(III) to ovotransferrin have been investigated using the stopped-flow technique. Rate constants for the second-order reaction, k ₊, were determined by monitoring the absorbance change upon formation of the metal-transferrin complex in time range of milliseconds to seconds. The N and C sites appeared to bind a particular metal ion with the same rate; thus, average formation rate constants k ₊ (average) were 2.4 × 10⁴ M^–1 s^–1 and 8.3 × 10⁴ M^–1 S ^–1 for Cu (II) and Tb (III) respectively. Site preference (N site for Cu (II) and C site for Tb (III)) is then mainly due to the difference in dissociation rate constant for the metals. Fe (III) binding from Fe-nitrilotriacetate complex to apo-ovotransferrin was found to be more rapid, giving an average formation rate constant k ₊ (average) of 5 × 10⁵ M^–1 s^–1, which was followed by a slow increase in absorbance at 465 nm. This slow process has an apparent rate constant in the range 3 s^–1 to 0.5 s^–1, depending upon the degree of Fe (III) saturation. The variation in the rate of the second phase is thought to reflect the difference in the rate of a conformational change for monoferric and diferric ovotransferrins. Monoferric ovotransferrin changes its conformation more rapidly (3.4s^–1) than diferric ovotransferrin (0.52 s^–1). A further absorbance decrease was observed over a period of several minutes; this could be assigned to release of NTA from the complex, as suggested by Honda et al. (1980).Abbreviations Tf ovotransferrin - NTA nitrilotriacetate Jichi Medical School, School of Nursing, Yakushiji 3311-159, Minamikawachi, Tochigi, 329-04 Japan     

Growth yield increase linked to caffeate reduction in Acetobacterium woodii

Growth yields were determined with Acetobacterium woodii strain NZva 16 on hydrogen and CO₂, formate, methanol, vanillate, ferulate and fructose in mineral medium in the absence and presence of 0.05% yeast extract. Yeast extract was not essential for growth but enhanced growth yields by 25–100% depending on the substrate fermented. Comparison of yields on formate or methanol allowed calculation of an energy yield in the range of 1.5–2 mol ATP per mol acetate formed during homoacetate fermentation of A. woodii. In the presence of 6 mM caffeate, growth yields were determined with the substrates formate or methanol. Caffeate was reduced to hydrocaffeate and increased growth yields were obtained. An ATP yield of about 1 mol per mol of caffeate reduced was calculated. Cytochromes were not detectable in cell free extracts or membrane preparations.     

Determination of isoprostaglandin F2α type III in human urine by gas chromatography–electronic impact mass spectrometry. Comparison with enzyme immunoassay

F₂-Isoprostanes are stable lipid peroxidation products of arachidonic acid, the quantification of which provides an index of oxidative stress in vivo. We describe a method for analysing isoprostaglandin F_2α type III (15-F_2t-IsoP) in biological fluids. The method involves solid-phase extraction on octadecyl endcapped and aminopropyl cartridges. After conversion to trimethylsilyl ester trimethylsilyl ether derivatives, isoprostaglandin F_2α type III is analysed by mass spectrometry, operated in electronic impact selected ion monitoring mode. We have compared enzyme immunoassay (EIA; Cayman, Ann Arbor, MI, USA) to this method with 30 human urine aliquots following the same extraction procedure in order to determine the agreement between both methods. Isoprostaglandin F_2α type III concentrations determined with gas chromatography–mass spectrometry (GC–MS) did not agree with those determined with EIA. Our results suggest that GC–MS and EIA do not measure the same compounds. As a consequence, comparison of clinical results using GC–MS and EIA should be avoided.     

A theoretical and experimental analysis of the qP and qN coefficients of chlorophyll fluorescence quenching and their relation to photochemical and nonphotochemical events

The initial (F₀), maximal (F_M) and steady-state (F_S) levels of chlorophyll fluorescence emitted by intact pea leaves exposed to various light intensities and environmental conditions, were measured with a modulated fluorescence technique and were analysed in the context of a theory for the energy fluxes within the photochemical apparatus of photosynthesis. The theoretically derived expressions of the fluorescence signals contain only three terms, X=J₂p_2F/(1–G), Y=T/(1–G) and V, where V is the relative variable fluorescence, J₂ is the light absorption flux in PS II, p_2F is the probability of fluorescence from PS II, G and T are, respectively, the probabilities for energy transfer between PS II units and for energy cycling between the reaction center and the chlorophyll pool: F₀=X, F_M=X/(1–Y) and F_S=X(1+(YV/(1–Y))). It is demonstrated that the amplitudes of the previously defined coefficients of chlorophyll fluorescence quenching, q_P and q_N, reflect, not just photochemical (q_P) or nonphotochemical (q_N) events as implied in the definitions, but both photochemical and nonphotochemical processes of PS II deactivation. The coefficient q_P is a measure of the ratio between the actual macroscopic quantum yield of photochemistry in PS II (41-1) in a given light state and its maximal value measured when all PS II traps are open (41-2) in that state, with 41-3 and 41-4. When the partial connection between PS II units is taken into consideration, 1-q_P is nonlinearily related to the fraction of closed reaction centers and is dependent on the rate constants of all (photochemical as well as nonphotochemical) exciton-consuming processes in PS II. On the other hand, 1-q_N equals the (normalized) ratio of the rate constant of photochemistry (k_2b) to the combined rate constant (k_N) of all the nonphotochemical deactivation processes excluding the rate constant k₂₂ of energy transfer between PS II units. It is demonstrated that additional (qualitative) information on the individual rate constants, k_N-k₂₂ and k_2b, is provided by the fluorescence ratios 1/F_M and (1/F₀)–(1/F_M), respectively. Although, in theory, 41-5 is determined by the value of both k_2b and k_N-k₂₂, experimental results presented in this paper show that, under various environmental conditions, 41-6 is modulated largely through changes in k _N, confirming the idea that PS II quantum efficiency is dynamically regulated in vivo by nonphotochemical energy dissipation.Abbreviations Chl chlorophyll - F₀, F_M and F_S initial, maximal and steady-state levels of modulated Chl fluorescence emitted by light-adapted leaves - PS I and II photosystem I and II - q_P and q_N (previously defined) photochemical and nonphotochemical components of Chl fluorescence quenching     

Synthesis and properties of red emitter Ru(II) complexes based on 6,6′-disubstituted-4,4′-bipyrimidine

Heteroleptic complexes [Ru(bpy)₂(R₂bpm)]²⁺, where bpy = 2,2′-bipyridine and R₂bpm = 6,6′-diaryl-4,4′-bipyrimidine, have been synthesized and characterized, together with the homoleptic complex [Ru(R₂bpm)₃]²⁺, in which R₂bpm = 6,6′-diphenyl-4,4′-bipyrimidine. The substituent aryl on the bipyrimidine has significant effects on the properties of these complexes as compared to the parent [Ru(bpy)₂(bpm)]²⁺ complex. The complexes exhibit Ru-to-bpm charge transfer (CT) absorptions centered at about 540 nm and Ru-to-bpy CT absorptions centered at about 435 nm. The assignment of the low energy absorptions is supported by the relative ease of the reduction of the new complexes as compared to [Ru(bpy)₃]²⁺. The new complexes exhibit a relatively intense emission at room temperature, with lifetimes in the 10-50 ns range, with the homoleptic species exhibiting the higher-energy (maximum at 724 nm) and the longest-lived (τ = 48 ns) emission among the complexes. Luminescence lifetimes and quantum yields are governed by the energy gap law, indicating that direct deactivation to the ground state is the dominant relaxation pathway for 1-6, while thermally activated processes are inefficient.     

Luminescent α-diimine complexes of ruthenium(II) containing scorpionate ligands

We describe the synthesis, characterization, and reactivity of several Ru(II) complexes of the type cis-L₂Ru(Z)ⁿ⁺, where L is an α-diimine [e.g. 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen)] ligand and Z is a bis-coordinated scorpionate ligand such as tris-(1-pyrazolyl)methane (HC(pz)₃, PZ=1-pyrazolyl; n=2) or tetrakis-(1-pyrazolyl)borate anion (B(pz)₄ ⁻; n=1). The complexes each exhibit strong visible absorption assigned as a π*(L)←dπ(Ru) metal-to-ligand charge-transfer (MLCT) transition characteristic of the cis-L₂Ru²⁺ kernel. A corresponding MLCT excited state emission is observed in room temperature CH₃CN solution, although emission energies, lifetimes, and quantum yields are reduced relative to Ru(bpy)₃ ²⁺. Electronic spectra and cyclic voltammetry measurements indicate that the relative π-acceptor abilities of the coordinated Z are: Z=(1H-pyrazolyl)₂(pz)₂B(pz)₂<(pyridine)₂<(pz)₂CH(pz). Uncoordinated pz groups of cis-(bpy)₂Ru(pz)₂B(pz)₂ ⁺ can be reacted to form a sterically hindered, localized-valence (K_com33 l mol⁻¹) cis,cis-(bpy)₂Ru^II(pz)₂B(pz)₂Ru^II(bpy)₂ ³⁺ dimer. The dimer properties are interpreted by comparison to the known cis-(bpy)₂Ru^II(pz)₂Ru^II(bpy)₂ ²⁺ analog. The dimer is photoreactive and undergoes an asymmetrical photocleavage in CH₃CN (yielding cis-(bpy)₂Ru^III(pz)₂B(pz)₂ ²⁺ and cis-(bpy)₂Ru^II(CH₃CN)₂ ²⁺), similar to the corresponding thermal reaction observed for the mixed-valence cis-(bpy)₂Ru^II(pz)₂Ru^III(bpy)₂ ³⁺ system.     

Determination of p‐Aminobenzenesulfonic acid based on the electrochemiluminescence quenching of tris (2,2’‐bipyridine)‐ ruthenium (II)

This study describes the quenching effects of p‐aminobenzenesulfonic acid (p‐ABSA) based on electrochemiluminescence (ECL) of the tris (2,2^’‐bipyridyl)‐ruthenium(II)(Ru(bpy)₃²⁺)/tri‐n‐propylamine (TPrA) system in aqueous solution. Quenching behaviours were observed with a 200‐fold excess of p‐ABSA over Ru(bpy)₃²⁺. In the presence of 0.1 M TPrA, the Stern‐Volmer constant (K_SV) of ECL quenching was as high as 1.39 × 10⁴ M^‐1 for p‐ABSA. The logarithmic plot of inhibited ECL versus concentration of p‐ABSA was linear over the range of 6.0 × 10^‐6 ‐3.0 × 10^‐4 mol/L. The corresponding limit of detection was 1.2 × 10^‐6 mol/L for p‐ABSA (S/N = 3). The mechanism of quenching is believed to involve an energy transfer from the excited‐state luminophore to a dimer of p‐ABSA and the adsorption of free radicals of p‐ABSA at the electrode surface that impeded the oxidation of the Ru(bpy)₃²⁺/TPrA system. Copyright © 2012 John Wiley & Sons, Ltd.     

Preparation, crystal structure and luminescent properties of lanthanide picrate complexes with N-benzyl-2-{2′-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-acetamide (L)

A new amide-based ligand derived from biphenyl, N-benzyl-2-{2′-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-aceamide (L) was synthesized. Solid complexes of lanthanide picrates with this new ligand were prepared and characterized by elemental analysis, conductivity measurements, IR and electronic spectroscopies. The molecular structure of [Eu(pic)₃L] shows that the Eu(III) ion is nine-coordinated by four oxygen atoms from the L and five from two bidentate and one unidentate picrates. All the coordinate picrates and their adjacent equivalent picrates form intermolecular π-π stacking. Furthermore, the [Eu(pic)₃L] complex units are linked by the π-π stacking to form a two-dimensional (2-D) netlike supramolecule. Under excitation, the europium complex exhibited characteristic emissions. The lifetime of the ⁵D₀ level of the Eu(III) ion in the complex is 0.22 ms. The quantum yield Φ of the europium complex was found to be 1.01 × 10⁻³ with quinine sulfate as reference. The lowest triplet state energy level of the ligand indicates that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.     

Loss of quantum yield in extremely low light

It has generally been assumed that the photosynthetic quantum yield of all C₃ plants is essentially the same for all unstressed leaves at the same temperature and CO₂ and O₂ concentrations. However, some recent work by H.C. Timm et al. (2002, Trees 16:47–62) has shown that quantum yield can be reduced for some time after leaves have been exposed to darkness. To investigate under what light conditions quantum yield can be reduced, we carried out a number of experiments on leaves of a partial-shade (unlit greenhouse)-grown Coleus blumei Benth. hybrid. We found that after leaves had been exposed to complete darkness, quantum yield was reduced by about 60%. Only very low light levels were needed for quantum yield to be fully restored, with 5 mol quanta m^–2 s^–1 being sufficient for 85% of the quantum yield of fully induced leaves to be achieved. Leaves regained higher quantum yields upon exposure to higher light levels with an estimated time constant of 130 s. It was concluded that the loss of quantum yield would be quantitatively important only for leaves growing in very dense understoreys where maximum light levels might not exceed 5 mol quanta m^–2 s^–1 even in the middle of the day. Most leaves, even in understorey conditions, do, however, experience light levels in excess of 5 mol quanta m^–2 s^–1 over periods where they obtain most of their carbon so that the loss of quantum yield would affect total carbon gain of those leaves only marginally.Abbreviations FBPase Fructose-1,6-bisphosphatase - RuBP Ribulose-1,5-bisphosphate - Rubisco RuBP carboxylase/oxygenase